Poet, Astrophotographer, Thinker.

If you, like me, an astrophotographer, you would have always wondered: “How do I know how much field framing the CCD or CMOS camera according to the focal telescope?“.

Well on the Internet there are several very complex calculations to have the answer to the question above.

You will ask now, “What site?” “How do you do it?”

In fact, it is very simple and with this blog I will briefly explain the operation.

The site in question is “DSA – Digital Sky Assist v1.0” and, as a title, will “assist you” to look for a deep sky object and will calculate for you the framed field (as the object will appear on the camera).

But we see step by step how to use this spectacular “raw material for astrophils”.

As a title of the example I will use this data:

Object searched: Messier 51, the whirlpool galaxy.

Telescope: Skywatcher 250/1200 (the data we need is 1200 mm or focal length);

1 .Connect to http://astro.xchris.net/dsa/, will come out of a screen like the image above (Image) where on the right will be written the rules for how to use the site and to the left the fields to fill in, where you will have to stop now.First, enter the name of the object in the “Search” field (in our case m51, then leave the rest as default),In fact the beauty of this site that, in addition to the name like our M51, can also be searched through astronomical coordinates or by its name “NGC xxxx” where “x” matches the object number;For coordinates just follow this rule: “ra xx xx xx dec xx xx xx”.Where the word “ra” stands for right ascension and instead of “x” you will enter the coordinates numbers in hours, minutes and seconds in order: “xx (hour) xx (mins) xx (second)”; therefore then follow the abbreviation “dec” which stands for declination and in place of “x” will put the numbers of coordinates in degrees, first and seconds in order: “xx (degrees) xx (first) xx (second)”. To make it easy we search for its name as a Messier object, so M51 as in Figure 1.

Figure 1

2 .Now in the field below (where the reflex camera is drawn) you have to enter the type of camera you are using.Here is the problem, because in the list there are not all the cameras in the world unfortunately, so you have to find one in the list that has the same sensor size as the sensor we use or approaching (just do a little research on google to know the size of the sensor, looking for a camera in the list and compare it to what you have); so according to our example we have a QHY163M that has a sensor size of about 17.7mm x 13.4mm, so we will have to look in the list of cameras what is approaching; for example, in our case the ASI1600MM COOL (sensor size 4: 3 or 17.5mm x 13.0mm) is worthy of being the sister, by sensor size, of our QHY163M, and finally we will choose it.Then also we choose if you check the final image rotation to 90 ° or less (try to see what comes out if checked) as in Figure 2.

Figure 2

3 . At the third point, underneath the camera field, you will find an additional field: “The type of telescope used”.In fact, the most important thing is the focal length of the telescope we use, in our case I will use a Skywatcher 250/1200, where 1200 mm is the total focal length of the telescope (remember to insert the effective focal length, so if you are going to use a focal reducer, you’ll need to include the focal difference from the native one, example: A native 80/500 refractor with a 0.8x reducer will have an actual focal length of 500 × 0.8 = 400 mm in total), which must be placed on the “Override Focal” text field below the list of telescopes; as far as the list of telescopes itself skips as well (leave default the first telescope in the list) because in my opinion there are so few, so it is not advisable to use it and only use the text field of the effective focal length that you can write directly to it by personalizing it.All this as in figure 3.

Figure 3

4 .At this point we select the NASA method with which our image will appear, there are different ones in the two lists, usually using “DSS” in the first list and “Linear” in the second, but sometimes you will also need to use “Logaritmic” in the second list for stand out, for example the nebula, if that is the object you were looking for; anyway I can guarantee that with most of the deep sky objects the “Linear” method is still good.Finally we will click “Search” as in figure 4.

Figure 4

5.Once processed, the object will appear (as you would already see in the camera) with all info (including coordinates).Just click on “Real Size” and you will see the object with the final calculation (as it will be shown in the camera, or its framed field), keep in mind that you will not expect 100% that will be as in the final figure when you are in the true operative field, however, i having made some tests, i assure you it works and that it get off only a little bit.Then you can also download the original “.fit” (given by NASA itself) by clicking on “Download Fit”, just below “Real Size”.All this as in figure 5 and 6.

Figure 5

Figure 6

Here is the “Real Size” of how an example M51 will be on a Skywatcher 250/1200 with the QHY163M (click on the picture to see the result):

Credits: NASA

Finally a note:I wanted to remember that the details are always the same, whether the focal is 500 mm for example, whether it’s 1200; what will change is the “zoom” of the image, as in reality it is a single big picture of the cube made in panels by the same NASA.

Hope it helps, helped me a lot and I use it often to compare.For more information, please leave a comment or a like or a star rating.